1. Field of the Invention
This invention relates to a semiconductor laser driving method and an optical disc apparatus, more particularly, to a semiconductor laser driving method for driving a semiconductor laser with a RF-modulated driving current and an optical disc apparatus using the driving method for driving a semiconductor laser used as a light source.
2. Description of the Related Art
As to semiconductor lasers used as light sources of optical pick-up devices in optical disc apparatuses, there are cases where a semiconductor laser is driven by a driving current made by modulating a D.C. current with a high-frequency (RF) current, that is, RF modulation of a semiconductor laser is done, for the purpose of reducing so-called optical feedback noise. In such cases, conventional technologies were configured to optimize the RF-modulating frequency, amplitude and/or waveform, depending on the optical system in which the semiconductor laser is used.
According to recent researches by the Inventor, in optical disc apparatuses used for reproduction of DVD-ROM (digital video disc ROM) with a much higher recording density than CD-ROM (compact disc ROM), a serious increase in jitter, probably caused by noise, was confirmed to occur at a specific optical output, regardless of sufficient RF modulation of an AlGaInP semiconductor laser used as a light source of an optical pick-up. Since such an abnormal increase in jitter invites deterioration of reading characteristics, a countermeasure therefor is desired.
This problem appeared in a process of reproduction of DVD-ROM with a high recording density. Essentially, however, it is presumably caused by the decreased pit sizes along with progressively high recording densities of optical discs. Taking an optical disc with the diameter of 12 cm, for example, the problem becomes apparent when its capacity is on the order of gigabytes. For example, the capacitor of an optical disc with the diameter of 12 cm for use with a semiconductor laser with the oscillation frequency of 650 nm (DVD-ROM, or the like) is 4.7 gigabytes, and the capacity of an optical disc with the diameter of 12 cm for use with a semiconductor laser with the oscillation frequency of 780 nm is 0.64 gigabytes (640 megabytes).
It is therefore an object of the invention to provide a semiconductor laser driving method which enables the use of a semiconductor laser under a low noise by preventing an abnormal increase in jitter at a specific optical output when the semiconductor laser is RF-modulated, and to provide an optical disc apparatus using the driving method.
The Inventor made active researches to overcome the above-indicated problems involved in the conventional techniques, as summarized below.
Here are repeated the problems involved in the conventional techniques in greater detail. The Inventor found an abnormal increase in jitter at a specific RF level, namely, at approximately 1.3 V (corresponding to the optical output of about 2.5 mW), in the dependency of jitter upon the RF level (linearly responding to the optical output of the semiconductor laser) obtained upon RF modulation of 350 MHz to an index-guided AlGaInP semiconductor laser with the oscillation frequency of 650 nm used as a light source of the optical pickup in a DVD-ROM optical disc apparatus. This phenomenon appeared under a condition with sufficient RF modulation and always appeared at the same RF level. Therefore, it was not a phenomenon caused by an optical feedback noise. Additionally, the same phenomenon was found also in various types of DVD semiconductor lasers.
FIG. 1 shows typical characteristics of optical output (L) to driving current (I) in both a case with RF modulation and a case without RF modulation. It is known from FIG. 1 that, without RF modulation, namely, in the D.C. driving mode, linear L-I characteristics appear; however, with RF modulation, the threshold current Ith decreases, and non-linear undulation occurs in the L-I characteristics. In terms of changes in differential quantum efficiency, the undulation is periodically attenuating fluctuation. Such undulation in L-I characteristics have not been understood heretofore.
The Inventor also found that the intrinsic noise (quantum noise) changed like xe2x80x9clumpsxe2x80x9d in substantial synchronism with undulation in the L-I characteristics. That is, although typical characteristics of relataive intensity noise (RIN) to average optical output (Pout) of a RF-modulated semiconductor laser are as shown in FIG. 2, the RIN-to-Pout characteristics shown here represent xe2x80x9clumpxe2x80x9d-like changes in the intrinsic noise approximately synchronizing with the undulation in the L-I characteristics. In FIG. 2, P1, P2, P3, . . . are average optical outputs in which RIN is maximized.
Then, as a result of observation of optical waveforms responsive to RF-modulation, it was found that a first pulse, second pulse and third pulse of relaxation oscillation occurred in synchronism with undulation in L-I characteristics. The start positions of respective pulses substantially coincide with the minimum positions (bottoms) in the L-I characteristics curve, and intrinsic noise derives from occurrence of a new oscillation mode near here. FIG. 3 shows the aspect of this phenomenon. In FIG. 3, D1 and D2 are bottoms in the L-I characteristics curve. Generation of intrinsic noise caused by occurrence of the new oscillation mode is principally a phenomenon similar to an increase in quantum noise caused by occurrence of kinks, and can be regarded as the origin of lumps in the intrinsic noise.
As explained above, it has been confirmed RF modulation of a semiconductor laser invites the phenomenon that the intrinsic noise periodically exhibits peaks at a specific optical output and that RF modulation of a semiconductor laser used as the light source of the optical pickup in an optical disc apparatus causes the problem of increasing the jitter at these peaks. The nature of the phenomenon can be briefed as follows.
Positions of lumps (positions of optical outputs) in the intrinsic noise in RIN-to-Pout characteristics are determined by the relation between relaxation oscillation frequency (fr) of the semiconductor laser and RF-modulation frequency (RF modulation cycle), RF-modulation waveform, RF-modulation amplitude, and so forth, and have the following natures.
(Nature 1) The larger the RF-modulation amplitude, the longer the cycle of changes in xe2x80x9clumpxe2x80x9d positions in the intrinsic noise.
(Nature 2) The higher the RF-modulation frequency (the shorter the RF-modulation cycle), the longer the cycle of changes in xe2x80x9clumpxe2x80x9d positions in the intrinsic noise.
(Nature 3) The higher the relaxation oscillation frequency, the shorter the cycle of changes in xe2x80x9clumpxe2x80x9d positions in the intrinsic noise.
(Nature 4) As the temperature becomes higher, the cycle of changes in xe2x80x9clumpxe2x80x9d positions in the intrinsic noise becomes slightly longer.
(Nature 5) When the feedback light increases, the noise amount increases and the cycle of changes in xe2x80x9clumpxe2x80x9d positions in the intrinsic noise becomes shorter.
(Nature 6) When the RF-modulation frequency becomes higher, the noise amount during low optical output decreases.
These natures derive from the following reasons. FIGS. 4A to 4C show changes in optical response to changes in D.C. bias current value in which the D.C. bias current value increases from FIG. 4A to FIG. 4B to FIG. 4C. When the D.C. bias current value is low (FIG. 4A), which represents the status where only the first peak of relaxation oscillation is output, the first peak of relaxation oscillation grows in response to the D.C. bias current. When the D.C. bias current value increases to represent the status shown in FIG. 4B where the effective pulse width Wp overlaps excitation of the second peak of relaxation oscillation, the second peak of relaxation oscillation starts to generate. It is known that a semiconductor laser, in general, generates intrinsic noise (quantum noise originating from spontaneous radiation) when a new oscillation mode begins to grow. The oscillation mode found this time is a new oscillation mode in the sense of time, that is, on the time base. As a phenomenon, however, it is considered to be an equivalent physical phenomenon in the sense that a new oscillation mode occurs, and it results in intrinsic noise being produced in response to the new oscillation mode. As the D.C. bias current value increases further, next noise occurs at positions where the third peak of relaxation oscillation appears (FIG. 4C). As the time progresses, these are repeated successively. In FIGS. 4A through 4C, Td denotes the delay time in oscillation of the semiconductor laser;
As explained above, although a RF-modulated semiconductor laser, in general, represents RIN-to-Pout characteristics having xe2x80x9clumpsxe2x80x9d in intrinsic noise, it is important to suppress xe2x80x9clumpsxe2x80x9d under a level practically acceptable for its optical output, namely, to RIN less than xe2x88x92130 dB/Hz, for example. For this purpose, there is the following countermeasure taking the above discussion into consideration. The basic concept of the countermeasure lies in:
(A) delaying generation of second pulses of relaxation oscillation (shifting them toward high outputs) to decrease the intrinsic noise produced there; or
(B) generating higher order pulses of relaxation oscillation earlier from the beginning (shifting them toward low outputs) to suppress generation thereof later.
If the practical optical output is relatively low than the specification of the semiconductor laser, then the approach (A) is more effective. If it is relatively high, the use of the approach (B) is more effective. If the semiconductor laser is desirably used both for a relatively low optical output and for a relatively high optical output, it is preferable to give preferential to the use for a relatively low optical output and to use the approach (A).
To realize (A), the following approaches may be employed. To realize (B), procedures opposite from the following approaches may be done.
(A-1) Enlarging the RF modulation amplitude.
(A-2) Increasing the RF modulation frequency.
(A-3) Decreasing the relaxation oscillation frequency of the semiconductor laser.
(A-4) Using a narrow rectangular wave is used as the RF modulation waveform.
When using (A-1), attention is called to the possibility of degrading the reliability due to excessive enlargement of first pulses of relaxation oscillation. Although (A-2) involves the same problem as (A-1), this can be overcome by setting the RF modulation amplitude slightly smaller. (A-2) is convenient because it contributes to suppression of the entire intrinsic noise as explained in (Nature 6).
(A-3) can be realized by increasing the edge surface reflectivity, increasing the cavity length, and/or reducing the quantum well effect. These approaches degrade the essential characteristics of the semiconductor laser and are not desirable. Nevertheless, if no other method is found, they are worth while to take into consideration. (A-4) is very effective, but involves the same problem as (A-1). However, in case of a high-output semiconductor laser whose edge surface is strong against breakage, it is worth while to use. The RF modulation waveform need not be strictly rectangular, but it is sufficient to be a short pulse relative to the relaxation oscillation frequency fr of the semiconductor laser.
The description made above has been directed to the reading process in the optical disc apparatus. Basically, however, the same applies also to the writing process.
According to the first invention of the present invention, there is provided a semiconductor laser driving method for driving a semiconductor laser with a driving current made by modulating a D.C. current with a high-frequency current, characterized in:
a curve representing relative intensity noise to average optical output characteristics of the semiconductor laser having at least one peak; and
the semiconductor laser being driven under a condition where the average optical output is offset from the peak.
According to the second invention of the present invention, there is provided an optical disc apparatus using a semiconductor laser as a light source thereof and configured to drive the semiconductor laser with a driving current made by modulating a D.C. current with a high-frequency current, characterized in:
a curve representing relative intensity noise to average optical output characteristics of the semiconductor laser having at least one peak; and
the semiconductor laser being driven under a condition where the average optical output is offset from the peak.
An example of the relative intensity noise (RIN) to average optical output (Pout) of the semiconductor laser according to the invention is as shown in FIG. 2. In FIG. 2, peaks P2 and P3 are lumps in intrinsic noise.
When the average optical output (recommended average optical output) used with the semiconductor laser is P*, conditions for modulation by RF current are typically determined so that peaks do not fall within P*xc2x1xcex94P (where xcex94P is the margin for the average optical output (recommended average optical output (see FIG. 2)), more specifically, within P*xc2x10.5 mW. In this manner, the semiconductor laser can be used with low noise by avoiding xe2x80x9clumpsxe2x80x9d in intrinsic noise while providing a practically ample margin for the efficiency of use of laser light. For example, as to optical disc apparatuses, an ample margin can be ensured to cope with variety among different lots of optical pickup devices, and the manufacturing yield can be increased significantly.
In order to use the semiconductor laser with low noise by avoiding xe2x80x9clumpsxe2x80x9d in the intrinsic noise when the average optical output used with the semiconductor laser is lower than 10 mW, the frequency fm of the RF current is determined to satisfy frxe2x89xa7fmxe2x89xa7fr/5 where fr is the relaxation oscillation frequency of the semiconductor laser. This corresponds to the approach (A-2) of the countermeasure (A) explained before.
In order to use the semiconductor laser with low noise by avoiding xe2x80x9clumpsxe2x80x9d in the intrinsic noise, a rectangular wave may be used as the driving current waveform, setting the pulse width Wp of the driving current to satisfy Td+1/frxe2x89xa6Wpxe2x89xa6Td+2/fr where Td is the delay time in oscillation the semiconductor laser. This corresponds to the approach (A-4) in the countermeasure (A) explained before.
In order to use the semiconductor laser with low noise by avoiding xe2x80x9clumpsxe2x80x9d in the intrinsic noise when the average optical output used with the semiconductor laser is higher than 10 mW, the frequency fm of RF current is determined to satisfy frxe2x89xa7fmxe2x89xa7fr/10 where the fr is the relaxation oscillation frequency of the semiconductor laser. This corresponds to the approach (A-2) of the countermeasure (A) explained before.
In case that the semiconductor laser is desired to practically use both for relatively low optical output and for relatively high optical output, if the RF modulation conditions are fixed to meet one of the requirements, the average optical output used to meet the other requirement is set to a value amply offset from peaks shown in FIG. 2.
Basically, the semiconductor laser used here may be of any type in terms of semiconductor materials, laser structures, structures of active layers, and so on. AlGaInP semiconductor laser and AlGaAs semiconductor lasers are specific examples thereof. If the active layer has a quantum well structure, there remain various choice in whether a strained quantum well structure is used or not, how much the strain should be, how the number of quantum wells and the thickness of well layers should be determined, for example.
According to the invention having the above-summarized construction, since the curve representing the relative intensity noise to average optical output characteristics of the semiconductor laser include at least one peak, and the semiconductor laser is driven under conditions ensuring average optical output offset from the peaks, the semiconductor laser can be used with low noise by avoiding xe2x80x9clumpsxe2x80x9d in intrinsic noise.